Guide to Portable CNC Plasma Cutting

• 1
  How the plasma arc removes metal
• 2
  Air, oxygen and nitrogen: what each gas does
• 3
  Thickness, amperage and speed
• 4
  What portable plasma can and cannot hold
• 5
  When plasma is the wrong process
• 6
  From plasma blank to finished part
• 7
  Matching amperage, gas and thickness

A plasma torch constricts an electric arc through a small nozzle while gas blows through the same opening. The gas heats into a fourth state of matter, ionized and conductive, and the arc transfers to the workpiece. Temperature at the arc core reaches roughly 22,000 °C, hot enough to melt steel instantly rather than burn it like oxy-fuel.

The molten metal is not vaporized away. The high-velocity gas jet pushes most of it out the bottom of the kerf. That is why cut quality depends so much on gas flow and standoff height: lose the jet and the dross stays on the part.

On a portable CNC plasma cutting table, a controller drives the torch along a programmed path from a CAD file. A cart-mounted gantry carries the torch, and the power supply, air filter and control box ride with it. The cut geometry is still CNC geometry. Only the machine base changed.

What actually limits accuracy on these systems is the rail and the plate, not the electronics. A gantry that flexes under acceleration, or a plate that is not flat, throws the torch off the programmed line by more than the controller ever will.

• 1
  Arc transfers to the workThe workpiece becomes part of the circuit, so it must be grounded.
• 2
  Gas does two jobsIt forms the plasma and clears molten metal from the kerf.
• 3
  Motion is still CNCSame G-code path as a fixed table, lighter machine frame.

Shop air is the default and it is fine for a lot of work. Clean, dry compressed air cuts mild steel up to about 12 mm with acceptable edge quality. The catch is the word clean. Moisture or oil in the line shows up as porosity on the cut face and short nozzle life, and it is the most common fault we see on field units.

Above roughly 12 mm on mild steel, or whenever the edge has to be weld-ready, oxygen from a cylinder or a gas console changes the result. Oxygen reacts with the iron and adds heat to the cut, which gives a squarer edge, tighter kerf and less dross. It also costs more per part and needs a second gas line to the torch.

Stainless steel and aluminum behave differently. Oxygen tends to leave an oxidized, hard edge on stainless. A nitrogen or argon-hydrogen mixture cuts cleaner on those alloys and on aluminum, though it needs higher flow and a gas console rather than a shop compressor.

The practical rule is simple. Match the gas to the alloy and the thickness, not to what is already plumbed in the shop. A 60 A air cut on 6 mm mild steel is a good part. The same setup on 20 mm stainless is scrap.

• 1
  AirMild steel to about 12 mm, cheapest, needs a dryer and filter.
• 2
  OxygenThicker mild steel, weld-ready edges, higher gas cost.
• 3
  Nitrogen or argon-hydrogenStainless and aluminum, cleaner edge, needs a console.

Amperage sets the ceiling on thickness. A 45–60 A unit cuts about 12 mm mild steel clean. An 80–100 A unit on a portable CNC plasma cutting machine handles up to roughly 25 mm mild steel, and stainless or aluminum to a lesser thickness because they conduct heat away faster. Always read the capacity chart for the exact torch, not the marketing line.

Cut speed matters as much as current. Run too slow and the arc dwells, widening the kerf and building dross on the bottom edge. Run too fast and the arc lags behind the torch, leaving a bevel and uncut ligaments at the end of the path. Each thickness has a window, usually a few hundred mm/min wide, where the dross is minimal and the bevel is small.

Thicker plate also means more heat input and more distortion. On a 20 mm part with long straight cuts, the plate can bow enough that the last cut runs at a different standoff than the first. Sequencing cuts to spread the heat, and clamping the plate down, fixes most of it.

A plasma cut edge carries a heat-affected zone, typically 0.5–2 mm deep depending on thickness and speed. That zone is harder than the base metal and can crack under fatigue if the part is cycled. If the edge is a functional surface, it needs machining, not just dressing.

• 1
  Too slowWide kerf, heavy dross, extra heat into the plate.
• 2
  Too fastLag lines, bevel, uncut tabs at the end of the path.
• 3
  Heat-affected zone0.5–2 mm of harder metal along the cut face.

Plasma is a thermal process, so the kerf is not a machined edge. A well-tuned portable system on 6 mm mild steel typically holds about ±0.5 mm on the path, with a kerf around 1.5–2 mm and a bevel of 2–5°. That is fine for brackets, base plates, gussets and weldments. It is not fine for a bearing bore or a mating face.

Hole quality is the weak point. Plasma holes under about 1.5× the plate thickness come out tapered and out of round, and the entry side is wider than the exit. If a hole has to take a bolt, drill it after cutting, or cut it undersize and ream. Many shops simply mark the hole with a pierce and let a drill do the work.

There is also a minimum feature size. Slots and tabs narrower than the kerf plus the heat-affected zone will distort or drop out. On 6 mm plate, keep webs at 3 mm or more. On 20 mm plate, think in terms of 8 mm and up.

This is where the process hands off to machining. Plasma produces a near-net blank. If a part needs ±0.005 mm, a Ra 0.8–1.6 μm finish, or a true bore, the blank goes onto a mill or a lathe afterward. Cutting the blank close to size saves material and cycle time.

• 1
  Path accuracyAbout ±0.5 mm on thin plate with a rigid rail setup.
• 2
  Kerf and bevelKerf 1.5–2 mm, bevel 2–5° on typical air cuts.
• 3
  Handoff pointTolerances under ±0.1 mm belong on a CNC mill.

Plasma is a cutting process, not a finishing process. It cannot produce a sealing face, a press fit or a threaded hole. If the drawing calls for a flatness of 0.05 mm over 300 mm, plasma will not hold it, and no amount of torch tuning will change that.

Thin sheet is another limit. Below about 1 mm, heat input warps the sheet faster than the torch can move. Laser or waterjet handles that range better. Plasma shines from roughly 3 mm up to 25 mm, where it is the cheapest way to get a blank.

Material also matters. Copper and brass conduct heat away so quickly that plasma cuts them poorly at typical amperages. Titanium cuts but needs shielding gas and a clean chamber to avoid contamination. For those materials, waterjet or a milling operation is usually the better call.

The strongest use case is a shop that needs a few hundred plate parts, in several thicknesses, with a short lead time. Plasma gets the blank cut in hours. Then the critical faces go to a 3-axis or 5-axis mill for the tolerances that matter. That split is cheaper than machining the whole part from solid.

• 1
  Good fitPlate 3–25 mm, weldments, brackets, one-off repair parts.
• 2
  Poor fitSheet under 1 mm, copper, high-conductivity alloys.
• 3
  Wrong fitSealing faces, press fits, bores, threads.

A plasma blank arrives with a hardened edge, a slight bevel and a kerf that is wider than the torch nozzle. Before machining, the edge usually needs a cleanup pass. A face mill or an end mill taking 0.3–0.5 mm off the cut face removes the heat-affected zone and gives a clean datum.

Setting the datum is the step that decides whether the finished part matches the model. Plasma edges are not square, so clamping against the cut face introduces error. Better to clamp against a rolled or sawn edge, or to probe the part on the machine and set work coordinates from the probed surface.

For parts with holes and pockets, the sequence we use is: cut the blank with a 2–3 mm allowance on machined faces, stress-relieve if the part is thick, then mill the datums, then the features. On a 5-axis machine the blank can often be finished in one setup, which keeps the position error inside ±0.005 mm.

Allowance planning is where a plasma shop and a machine shop have to talk. Too little allowance and the cutter hits hard spots in the heat-affected zone. Too much and you spend cycle time removing material that a better nesting plan could have avoided.

• 1
  Cleanup pass0.3–0.5 mm off the cut face to remove the HAZ.
• 2
  Datum choiceClamp or probe a non-plasma surface when possible.
• 3
  AllowanceLeave 2–3 mm on faces that will be machined.